[cdc] Do not check referenced field if no physical fields provided when computing columns

[JackeyLee007]

Purpose

Linked issue: close #xxx

This PR is for computing columns when using kafka_sync_database. There would be multiple different tables to sync, so it's hard to list the exact data fields. No data fields provided would stop the buidling of computing column expressions.

This PR ignores the check when building expressions for kafka_sync_database, since it provides empty_list as the data fields. And it postpones the check to evaluation, which returns null if the referenced field doesn't exist.

Tests

API and Format

Documentation